1. Technical Field
The present invention relates to a crystal element manufacturing method in which a crystal element is manufactured while the frequency of the crystal element is maintained at a predetermined precision and is efficiently adjusted by means of wet etching, in particular, by means of partial etching at a wafer level (wafer method), and a crystal resonator manufactured by this method.
2. Background Art
In recent years, crystal resonators, which have superior frequency stability with no variation with respect to a reference frequency, have been widely used as a reference source of frequency and time in electronic devices including many types of communication devices.
This type of a crystal resonator is formed from a crystal wafer or the like that is cut out from an artificial crystal by, for example, an AT cut (generally a cut out at an angle of approximately 35° 15′ with respect to the Z axis), and is then formed by means of mechanical polishing and wet etching or dry etching so that the crystal element maintains its thickness according to a desired frequency.
In general, in an AT cut crystal element, thickness-shear vibration is applied to the vibration mode thereof, and the vibration frequency thereof is in reverse proportion to the thickness of the crystal element, with the vibration frequency becoming higher as the thickness of the crystal element becomes smaller.
Incidentally, in recent years, highly notable development is being made in achieving higher frequencies of crystal resonators, and the thickness of a crystal element is increasingly being minimized due to this achievement of higher frequency. Consequently, a high level of processing precision and a high level of productivity are required in order to maintain the thickness of a crystal element at an appropriate thickness in manufacturing. If the processing precision does not meet a certain standard, then a problem will occur in the frequency adjustment of the crystal element, and productivity of the manufacturing will be reduced.
For example, in a crystal resonator manufacturing method disclosed in Patent Document 1 (Japanese Unexamined Patent Publication No. 2001-102654), first, as shown in FIG. 4 and FIG. 5, a crystal wafer 1 is prepared, and a large number (for example, 1,000 or more) of so-called reverse mesa sections respectively having a step section 2a, a partition 2b, and a bottom section 2c, which will become crystal element formation sections, are formed, by means of wet etching or the like, only on the front surface 2 among the front surface 2 and back surface 3 of the crystal wafer 1.
The crystal wafer 1 having a large number of the reverse mesa sections formed thereon in this way, is mounted on a crystal resonator manufacturing apparatus as shown in FIG. 6, to thereby adjust the frequency thereof. That is to say, with the step sections 2a formed on the crystal wafer 1 facing upward, prior to wet etching, partition plates 10 are bonded and fixed in a standing condition on the front surface 2 of the crystal wafer 1 with use of an adhesive agent or the like, in order to prevent etchant from flowing into the adjacent step sections 2a. Then, etchant 16 is dropped in a sequential manner into the respective step sections 2a from a nozzle (syringe) 15a of a dispenser 15 via through holes 11 provided in the partition plates 10 to thereby perform etching, and then neutralizing liquid or the like is dropped into the step sections 2a from a separately provided dispenser (not shown in the drawing) to thereby stop progression of the wet etching.
Next, the frequency of a vibrating section (crystal element formation section) formed within the step section 2a is measured using a separately provided frequency measuring apparatus, and is compared with a reference frequency, and if a deviation is present, the thickness of the vibrating section (step section 2a) will be reduced according to the deviation to thereby make an adjustment to achieve the desired frequency.
However, in such a conventional method of adjusting the frequency of a crystal resonator, the etchant 16 is individually dropped into each of, for example, 1,000 or more step sections (crystal element formation sections) formed on a crystal wafer to make frequency adjustments, and it is absolutely necessary to suppress variation in frequency of each crystal element in order to stabilize the frequency characteristics of the crystal elements. Consequently, frequency adjustment is performed for each crystal element in two stages of adjustment, namely coarse adjustment [for example, up to 200 ppm of reference (target) frequency] and fine adjustment [for example, up to 50 ppm of target frequency].